Ultrastructural studies of neurosecretion in Periplanta americana, L. : with special reference to the optic lobe and to ancillary circulatory structures.

This thesis presents mainly ultrastructural data on neurosecretory (NS) cells in the optic lobe and on several ancillary circulatory structures. A vital staining technique for NS material using acridine orange is described. All known types of NS material in insects give a positive reaction with this technique. The ultrastructure, histology and histochemistry of a new group of NS cells in the optic lobe are described. The axons of these cells are innervated by axons containing small dense granules which probably control the function of the NS cells. The optic lobes have been previously postulated as the site of a circadian clock controlling locomotor activity and the NS cells may be intimately involved with this clock mechanism. Initial experiments to find a causal relationship between the NS cells and activity were equivocal. Four pairs of segmental blood vesseIs occur in association with abdominal excurrent ostia of the heart. The segmental blood vessels are composed of fibroblasts and connective tissue strands and have numerous longitudinal NS axons which form an extension of the neurohaemal organ 1 associated with the lateral cardiac nerve. The number of release sites in the NS axons show a marked diurnal variation. The valves of the segmental blood vessels are composed of a peculiar type of muscle. They are innervated by granule-containing axons. The valves have a rhythm different to the heart, and it appears as if they have different controlling mechanisms. The valves appear to control the distribution of blood to various regions of the abdomen. The NS axons of the lateral cardiac nerve are at least partly derived from segmental nerves of the ventral cord as well as from the median/transverse nerves. There is a pair of peripheral NS cells at the junction of the median/transverse nerve and the link nerve. There is also a multipolar neuron in this region. The link nerve appears to constitute a neurohaemal site. Accessory pulsatile organs associated with the antennae have a complex structure. Briefly, two ampullae are attached to the Irons and they are connected to each other by a pulsatile muscle. There is a neurohaemal system in the ampulla wall which contains terminal Herring bodies. A vessel runs to the antenna from the ampulla. The cells of the vessel wall are specialized and have many resemblances to other epithelia involved in active transport of ions and/or water.